1. Field of the Invention
This invention relates to a display method for a high-gradation displaying operation in an electro-optical display device constructed by plural picture elements which are arranged in a matrix form and have driving switch elements, such as a liquid crystal display, a plasma display, a vacuum microelectronics display and the like.
2. Description of Related Art
The recent miniaturization of various office automation equipments has caused a conventional cathode ray tube (CRT) to be replaced by a thin-type display (flat panel display) such as a plasma display, a liquid crystal display and the like. In addition, there has been also researched a vacuum microelectronics display in which micro vacuum tubes each comprising a field emission cathode and a grid are arranged in a matrix array and an image is displayed by irradiating an electron beam emitted from the matrix array onto fluorescent material. In all the display devices as described above, an image display operation is performed by controlling a voltage to be applied to intersections of the matrix array.
That is, a transmitted-light amount or a scattered-light amount is varied by an electric field in a display of liquid crystal material, an electric discharge is induced between electrodes by an electric field in a plasma display, and electrons are emitted from a cathode by field emission effect in a vacuum microelectronics display.
The simplest one of these matrix types is a display including a pair of substrates which are confronted to each other, and striped wirings which are arranged longitudinally and laterally on the respective substrates, a voltage being generated in a gap between any intersected longitudinal and lateral wirings by applying a voltage therebetween. This type is called as a simple matrix-structure. This type of display can be produced easily and at low cost because of its simple structure. However, in this type of display, there has been frequently occurs a phenomena called as crosstalk in which an image is blurred due to unintentional signal flow into undesired parts in a driving operation of the display. In order to avoid the crosstalk, material whose optical characteristic varies sharply with a voltage above a predetermined threshold voltage is required. For example, a plasma electric discharge display is a favorable display for such a simple-matrix system because it has a distinct threshold value as described above.
When such an optical material as described above is used, however, the display must be driven such that a voltage for each picture element (that is, a crossing between matrix wirings) is extremely near to the threshold voltage. Therefore, when the simple matrix system is adopted, an optical ON/OFF-switching operation can be carried out, but it is difficult to obtain an intermediate brightness or color tone because material which can. vary its brightness in an intermediate variable range in accordance with an applied voltage can not be used as an optical material for the display.
This problem is caused by placing the switching function on an optical material (liquid crystal or electric discharge gas). Therefore, an attempt of installing a switching element to the matrix independently of the optical material was tried. This type of device is called as an active matrix display and has one or more switching elements at each picture element. A PIN diode, an MIM diode or a thin film transistor or the like is used as a switching element.
However, even though an active matrix system is adopted, it is difficult to achieve a display operation with high gradation as realized in CRT.
FIG. 1(A) shows a conventional gradation display system. In FIG. 1(A), the ordinate represents the amplitude of a voltage applied to a specified picture element and the abscissa represent a time, and this figure represents the variation of the voltage applied to a picture element of a liquid crystal display. The voltage is applied in the form of an alternative current pulse because the liquid crystal would be deteriorated due to its electrolysis if it is applied with a direct current for a long time.
In this figure, the voltage is applied so as to display brightness of "8" in first two periods, "4" in next one period and "6" in last one period. Actually, the liquid crystal material varies in its optical characteristic sharply at a particular threshold value, but it is assumed here that the optical characteristic varies linearly in accordance with the applied voltage. This approximation is a very close approximation for the liquid crystal material such as dispersion type liquid crystal material for example. Thus, in order to achieve the display operation with 16-step gradation for example, it is required to control a voltage at 16 steps and then apply it to a picture element.
In a usual liquid crystal material, its optical characteristic is saturated when applied with a voltage over 5 volts, and hardly varies even if a voltage above 5 volts is applied. In order to implement 16-step gradation displaying operation for example, a voltage must be applied with precision of 300 mV which is obtained by dividing 5 volts by 16. It is reasonable that the implementation of a higher-gradation display operation requires a more minute voltage to be applied to the picture element. However, it is not easy to generate a voltage with a resolution of 300 mV or less, and such a minute voltage is attenuated by various factors until it reaches the picture element. These factors contain resistance of wirings, resistance of thin film transistors, reduction of potential of a picture element due to a parasitic capacitance of the thin film transistors and the like. Since these parameters causing the voltage variation or fluctuation are different in accordance with an active element of each picture element, the fluctuation of the voltage of the picture element can be actually suppressed in a range of plus and minus 0.2 V at maximum over the whole panel.
On the other hand, there is another method of implementing a gradation displaying operation by controlling a time length (retention time) of a voltage pulse to be applied to each picture element. For example, display methods as disclosed in Japanese patent application Nos. 3-169306, 3-169307, 3-209869, etc. which have been invented by the same inventors as this application are cited as examples of the above method. FIG. 1(B) shows this example. First two periods are used for brightness of "8", next one period is used for brightness of "4" and last one period is used for brightness of "6", as well as the method of FIG. 1(A).
It is known that the liquid crystal material visually functions to display color tone and brightness in accordance with, not an instantaneous voltage, but an average effective voltage. Namely, assuming an effective voltage of first two periods as 1, the next one period is considered as 0.5 though it has the same peak voltage as that of the first two periods, and the last period is considered as 0.75.
Further, a response speed of the plasma electric discharge is a high speed of 1 micro second, but a human naked eye cannot follow such a high speed, and can sense only an average brightness, so that a visual brightness is finally determined by an average effective voltage.
That is, the gradation displaying system as described above requires the switching speed to be remarkably increased particularly in order to implement a high-gradation displaying operation.
FIG. 2 shows a special case of FIG. 1(B), and an example of FIG. 2 can achieve 64-step (64-level) gradation displaying, operation. Numbers at the left side represent degree of brightness of picture elements. In this example, the optical characteristic varies from "1" to "54" in this order. In FIG. 2, (A) and (B) are not different essentially, and only the order of plural pulses is altered therebetween. The details of this example are described in Japanese patent application No. 3-209869 which has been invented by the same inventors as this application and thus the description thereof is eliminated.
For example, in a part marked as "17", a pulse whose length is 1 and a pulse whose length is 16 appear once in a period of s respectively, and it represents an average brightness of "17". Further, in a part marked "37", a pulse whose length is 1, a pulse whose length is 4 and a pulse whose length is 32 appear once in a period of s, and it represents an average brightness of "37". By this way, 64-step gradation display from "0" to "64" can be achieved.
It is apparent from FIG. 2 that the minimum pulse length is required to be one 64th of a voltage repetitive period of s. In a case where a switching operation is actually carried out using a thin film transistor or the like, a pulse whose width is shortened in accordance with the number of lines of matrix is applied to the thin film transistor. For example, when the matrix has 480 lines, a pulse whose width is one 480th of the minimum pulse length is applied to the thin film transistor. Since s is usually 30 msec, the minimum pulse width becomes 500 micro sec. Thus, 1 micro sec is required for a driving signal for the thin film transistor or the like. This value may be considered as a large value, but it is very rapid signal for the thin film transistor. Therefore, in order to achieve higher gradation displaying operation, more rapid pulses must be applied, and by this, electromagnetic wave is radiated from the display.